1. Field of the Invention
The present invention relates generally to improved cross-linked aqueous well treating fluids and methods for preparing and using such fluids for treating subterranean zones in wells.
2. Description of the Prior Art
High viscosity aqueous cross-linked gels are used in a variety of operations and treatments in oil and gas wells. Such operations and treatments include, but are not limited to, well completion operations, fluid loss control treatments, production stimulation treatments, formation permeability conformance operations, and treatments to reduce water production.
An example of a production stimulation treatment utilizing a high viscosity cross-linked gelled fluid is hydraulic fracturing. In hydraulic fracturing treatments, the high viscosity fluid is utilized as a fracturing fluid and also carries particulate propping agents, e.g., sand, into the fractures formed. That is, the fracturing fluid is pumped through the wellbore into a formation to be stimulated at a rate and pressure such that fractures are formed and extended in the formation. The propping agent is suspended in the fracturing fluid so that it is deposited in the fractures when the gel is broken and returned to the surface. The propping agent functions to prevent the formed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the wellbore.
An example of a well completion operation involving the use of a high viscosity aqueous cross-linked gel is gravel packing. In gravel packing operations, solid gravel particles such as sand are carried to the subterranean zone or formation in which a gravel pack is to be placed by a high viscosity cross-linked gelled fluid. That is, the gravel is suspended in the high viscosity fluid at the surface and carried to the subterranean zone or formation in which the gravel pack is to be placed. Once the gravel is placed in the zone or formation, the cross-linked gel is broken (degraded) and returned to the surface. The gravel pack produced functions as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and up the wellbore.
Borate ion has long been used as a cross-linking agent for forming high viscosity cross-linked gelled aqueous well treating fluids. Various sources of borate ion have been utilized including boric acid, borax, sodium tetraborate, slightly water soluble borates such as ulexite, and other proprietary compositions comprising boric acid and dimers and trimers of borate ions. Numerous sources of borate cross-linking solutions are known in the art. U.S. Pat. No. 5,310,489 issued to Sharif on May 10, 1994; U.S. Pat. No. 5,266,224 issued to Sharif on Nov. 30, 1993; U.S. Pat. No. 5,252,236 issued to Sharif on Oct. 12, 1993; and U.S. Pat. No. 5,160,445 issued to Sharif on Nov. 3, 1992, which are incorporated herein by reference, disclose borate cross-linking solutions that may be useful in the present invention.
Polyvalent metal cations have also been used as cross-linking agents to form gelled aqueous well treating fluids. U.S. Pat. No. 5,145,590 issued to Dawson on Sep. 8, 1992, discloses that polyvalent metal cations including aluminum, antimony, zirconium and titanium containing compounds have been used to cross-link polymers used as well treating fluids. U.S. Pat. No. 4,514,309 issued to Wadhwa on Apr. 30, 1985, discloses a cross-linking system based on titanium and boron, including organotitanate compounds.
For well treating fluids to function properly, the fluids must remain viscous until the desired well operation or treatment has been completed. For example, the well treating fluids must remain viscous in a fracture long enough to permit build-up and maintenance of sufficient pressure to open a fracture and the fluid must retain its viscosity as it flows into the fracture to support the propping agents suspended in the fluid. Similarly, the well treating fluids must remain viscous in a gravel packing operation until the gravel is in place. Often, however, prolonged exposure of a well treating fluid to the temperatures encountered in the well causes the fluid to prematurely degrade and lose its viscosity. Many such fluids lose significant viscosity in a few hours at the temperatures experienced downhole, some in the range of from about 150.degree. F. to 400.degree. F., but usually in the range of from about 200.degree. F. to 300.degree. F. Preferably, a well treating fluid should retain its viscosity for up to 8 hours at temperatures of about 200.degree. F. to 300.degree. F.
There is, therefore, a need for improved cross-linked well treating fluids that can retain their viscosity for prolonged periods of time and methods for preparing and using such fluids for treating subterranean zones in wells.